Hydraulic fracturing is a technology commonly used to enhance oil and gas production from a subterranean formation. During this operation, a fracturing fluid is injected through a wellbore into a subterranean formation at a pressure sufficient to initiate fractures in the formation. Following the fracture initiation, particulates commonly known as proppants are transported into the fractures as a slurry, that is, as a mixture of proppant suspended in fracturing fluid. At the last stage, fracturing fluid is flowed back to the surface leaving proppants in the fractures, forming proppant packs to prevent fractures from closing after pressure is released. The proppant packs provide highly conductive channels for hydrocarbon to effectively flow through.
There are a number of proppants, including sands, ceramic particulates, bauxite particulates, glass spheres, resin coated sands, synthetic particulates and the like. Among them sands are by far the most commonly used proppants. Proppants normally range in size from 10 to 100 U.S. mesh, which is about 150 to 2,000 μm in diameter.
A vast majority of the fracturing fluids currently used are aqueous-based. Since proppants normally have a significantly higher density than water, for example the density of sand is typically about 2.6 g/cm3 while that of water is 1 g/cm3, high viscosity fluid is required to prevent proppants from settling out of the slurry. For this purpose, water-soluble polymers or viscoelastic surfactants are commonly added into the slurry to increase the fluid viscosity. From a scientific point of view, when a water soluble polymer such as guar gum polymer or one of its derivatives is dissolved in an aqueous liquid, the physical entanglement of polymer chains leads to a significant increase in the fluid viscosity. When polymer chains are further chemically linked by certain chemical compounds known as crosslinkers, forming so-called cross-linked gels, the viscosity is further enhanced. Guar gum cross-linked by borates is a well-known example of this technology in the fracturing industry. In comparison with a cross-linked fluid, linear gels, i.e., fluids containing enough polymer to significantly increase fluid viscosity without cross-linking, cause less formation damage and are more cost-effective, but have relatively poor suspension capability.
Water or slick water, i.e., water containing a very small amount of friction reducing agent, ranging roughly from 0.02% to 0.05% of the fluid, is widely used in fracturing shale or tight formations. Different water-soluble polymers, including guar gum and its derivatives, polyethylene oxide (PEO) and polyacrylamide and its derivatives, can be used as friction reducing agents. Polyacrylamide-based friction reducing agents, which includes polyacrylamides and polyacrylamide copolymers (which contain other monomers in addition to acrylamide monomers), are predominantly used in hydraulic fracturing operations.
Because of its low cost and ability to create a complex fracture network leading to better production, water or slick water has recently become the “go-to” fluid for fracturing shale or tight formations. However, it does not have sufficient viscosity to effectively transport proppants. Poor proppant transport is the single largest disadvantage in using slick water, as much more water is required to transport the same amount of proppant as would a more viscous fluid, to effectively prop open the fracture networks. The amount of water needed to perform a slick water fracturing job typically exceeds 15,000 to 30,000 m3.
Water usage and flowback water management in shale fracturing pose serious environmental challenges. To overcome these disadvantages, U.S. Pat. Nos. 7,723,274 and 8,105,986 to Applicant, incorporated by reference herein in their entirety, teach a composition and method for making stable aqueous slurry to efficiently transport proppants. By rendering the proppant surface hydrophobic, micro-bubbles are attracted and attached to the surface, making the proppant float in the slurry, resulting in an exponential increase in the water's ability to transport proppant without changing fluid rheology.
Not only is proppant transport important, but once proppants are transported into fractures it is highly desirable that they remain in the fractures after the fracturing operation. Unfortunately, it is not unusual that significant amount of proppant is dislodged and entrained in the fracturing fluid that flows out of formation. This process, known as “proppant flowback” in the industry, not only causes undue wear on production equipment but also reduces fracture conductivity. One of the most common approaches for reducing proppant flowback is to use resin coated proppants, normally as tail-in, in the last proppant placement stage. Another method, as disclosed in U.S. Pat. No. 6,047,772, is to use tackifying compounds to coat the proppant grains making them tacky so that they stick together, to retard proppant flowback. Both of these approaches are expensive and usually operationally challenging.
There is interest, particularly in the oil and gas industry, for compositions and methods for treating of particulates, such as proppants, that are more efficient and cost effective.